1. Field of the Invention
The present invention relates to an ultrasonic microscope intended to scan that face of a sample which is to be viewed through the microscope with an ultrasonic beam and, more particularly, it relates to an ultrasonic microscope intended to use, as a coupler liquid, a low temperature liquid such as nitrogen, argon and helium liquids.
2. Description of the Related Art
The ultrasonic microscope which has been recently developed attracts attention these days as a third microscope following the optical and electronic ones. The ultrasonic microscope can obtain the image of a sample in such a way that an ultrasonic beam is focused onto the viewed face of the sample by means of an acoustic lens to two-dimensionally scan the face of the sample. When an ultrasonic wave of high frequencies (ranging from several hundreds MHz to several GHz) is used to increase resolving power in the case of this microscope, the interval between the acoustic lens and the sample must be made smaller as the frequency of the ultrasonic wave becomes higher because the acoustic absorption ratio of coupler liquid (or water) is proportional to the square of frequency. The interval between the acoustic lens and the sample is defined by the curvature radius of the acoustic lens. An acoustic lens having a small radius of curvature must be used to shorten the interval. In a case where the frequency of ultrasonic wave used is several GHz, for example, the curvature radius of acoustic lens must be made smaller than several tens .mu.m. It is extremely difficult to make an acoustic lens which has such a finely curved surface. Even if such acoustic lens can be made, it will raise the cost of the microscope.
To enhance resolving power without setting the curvature radius of acoustic lens to the above-mentioned value, there has been provided another supersonic microscope (which will be hereinafter referred to as a low temperature ultrasonic microscope) intended to use, as coupler liquid, a low temperature liquid having a lower sound speed than that of water, such as nitrogen, argon and helium liquids and to scan the viewed face of sample in this low temperature liquid.
According to the low temperature ultrasonic microscope disclosed in U.S. Pat. No. 4,920,803 for example, however, the sample rod arranged freely movable up and down in the gas-tight chamber must be pulled completely out of the chamber in a case where the sample is to be exchanged with a new one. The sample rod is sealed by an O-ring and it must pass through the O-ring when it is to be pulled completely out of the gas-tight chamber. This makes it necessary for the sample rod to have a same radius all over its whole length. In other words, such a focusing system as disclosed in U.S. patent application Ser. No. 07/399,318 and having a radius larger than that of the sample rod could not be attached to the lower end of the sample rod.